Present shape memory polymers are materials that can memorize one or two temporary shapes and eventually revert to an original permanent shape upon exposure to an external microstructural-transforming stimulus such as heat. In some shape memory polymers the external stimulus for shape change may be an electric or magnetic field, light, or a change in pH.
A conventional shape memory polymer (SMP) is deformed at an elevated temperature (deformation temperature, Td) and the deformed temporary shape is fixed upon cooling. Often, this deformation temperature is above the glass transition temperature of the polymer composition. When heated to a recovery temperature (Tr), the temporary shape reverts to the original permanent shape. With a total of two shapes involved in each shape memory cycle, such an effect is called dual-shape memory effect (DSME) where the two shapes consist of the deformed temporary shape and the permanent shape. Quantitatively, this effect is evaluated based on the percentage of shape fixation of the temporary shape (shape fixity Rf, i.e. strain imposed compared to strain retained) and shape recovery of the permanent shape (shape recovery, Rr).
At the molecular level, materials displaying the DSME typically possess a polymer microstructural mechanism for setting the permanent shape and a reversible polymer phase transition for fixing the temporary shape. A prototype shape memory cycle occurs with both shape fixation and recovery above a reversible phase transition temperature (or the shape memory transition temperature, Ttrans).
In contrast to polymer materials displaying a dual-shape memory effect, a triple-shape memory effect has also been observed in some polymers. The triple-shape memory effect refers to the capability of some combinations of polymer materials to memorize a second temporary shape (three shapes are involved) using an additional reversible phase transition in the polymer composition. The fixation of two temporary shapes in a body of the polymer (and subsequent shape recovery) for a triple-shape memory polymer is achieved either above or between two transition temperatures existing in the mixed polymer composition.
Overall, various SMP systems have been adapted for use in a number of very useful applications including biomedical devices, self-healing surfaces, “smart” fasteners, and “smart” adhesives. In each of these applications the polymer may be placed in a temporary shape for initial placement. But upon heating (or other application of energy) the polymer self-transforms from its temporary shape to its permanent shape. It is apparent that the ultimate potential of this class of materials hinges heavily on tailoring (or tuning) their shape memory properties for the targeted applications. Due to the strong tie between Rf (and Rr) and Ttrans, tuning shape memory properties often involves adjustment in Ttrans, which requires material composition change via synthesis of new polymers or modification of existing polymers.
There remains a need for the adaptation of new polymer materials and new uses of polymer materials in SMP applications.